For medical diagnosis, treatment, and monitoring, it is often necessary to measure relevant molecules from biological tissues and fluids. It has long been desirable to make these chemical measurements without invasive procedures, like withdrawing blood. Implantable devices have been developed using electrochemical sensors embedded beneath the skin to measure the concentration of relevant chemicals in blood or interstitial fluid. For example, implantable devices have been developed to measure blood glucose in a diabetes mellitus patient.
These devices suffer from several deficiencies. One significant problem is a power source necessary to drive the sensing element and process signals obtained therefrom into useful information. An obvious power source for such implantable devices is batteries. Batteries have a number of shortcomings, however. High-capacity batteries exhibiting long life are large and bulky, and therefore not ideal for long-term implantation. Lowering battery capacity will decrease the size of the implanted unit, however useful life of the device will also be shorter. Replacement of an implantable device for any purpose may require surgery, therefore more frequent invasive procedures may be required to change batteries more often. In addition, the electrochemistries responsible for battery function are often based on hazardous and/or toxic substances like mercury or cadmium that pose a risk of injury to the patient. The problem with power sources for such implanted sensing devices is further intensified when the electrochemical sensing platform becomes a closed-cycle system—i.e., is reversible to allow sensing without consumption of any active agent. When a closed-cycle sensing chemistry is included, the need for long-lived implanted power systems is even more acute.
Therefore it is desirable to develop an implantable device that can power a sensor without the use of batteries. In the prior art, U.S. Pat. No. 7,125,382 (Zhou et al.), incorporated herein by reference in its entirety, describes an implantable biosensor system that uses radio frequency identification (RFID) technology, including a remote transponder that is in wireless communication with a passively powered on-chip transponder. The system is specifically adapted to provide a substantially stable and precise voltage to a sensor assembly that is included with an implantable on-chip transponder. The remote transponder is placed within a predetermined distance of the on-chip transponder in order to supply power to and request telemetry data from the on-chip transponder. The remote transponder is also configured to remotely receive data representative of a physiological parameter of the patient as well as identification data and may enable readout of one or more of the physiological parameters that are measured, processed and transmitted by the on-chip transponder upon request by the remote transponder.
However, devices in the prior art are typically based on electrochemical sensors. A physical sensor that functions without the need for batteries is needed.